The use of a potentiometer to determine the position of a connected device has proven to be rather accurate. The potentiometer employs either a rotary or a linear slide member connected to a variable resistor that changes resistance to be equated to the position of the device that is monitored. If the travel of the monitored device is greater than the travel of the slide member, gearing has been used to match the travel of the slide member to the travel of the monitored device. This application is accurate as long as the two devices are mechanically attached so that each increment of movement of the monitored device has a resistance change in the potentiometer and each position of the monitored device has a resistance value that is always the same for that position. If the devices are separated, for service or repair, the slide member must be reset to match the original position of the monitored device to the original resistance value for that position. Some applications can use a setup routine to move the slide member to the proper position, but if the potentiometer is not in the correct position at the beginning of the setup routine, the drive can bottom out or exceed the travel of the slide member before the correct resistance is found. When this occurs, the potentiometer can be damaged and must be replaced.
Known potentiometers may have dead spots or areas of no resistance change at either end of their travel. These dead spots can be misinterpreted by the control monitoring the potentiometer, as a no-movement condition of the potentiometer even though the slide in the potentiometer is continuing to move. A control unit can be programmed to recognize this condition as the movement limits of the monitored device, but the potentiometer must be preset to the correct position for this to be true. Being able to automatically disengage the potentiometer drive is necessary to prevent damage and if the potentiometer is enclosed and not readily accessible, it is necessary to have the ability to automatically reset the potentiometer travel to be within the travel of the monitored device and program the controls to accept the resistance output as the movement profile of the monitored device. Various attempts have been made to overcome the aforementioned problems as evidenced by the following patents.
U.S. Pat. No. 4,004,264 to Hogue et al. shows a variable resistance device of the type wherein a worm screw is utilized to move a contact bearing member through a path of travel relative to conductive and resistive tracks. The variable resistor includes a ratchet member having a single elongated spring section attached to a body portion. The body portion has two rigid legs extending therefrom. The spring portion of the ratchet member is disposed within a groove formed in a surface of the contact bearing member. The rigid legs of the ratchet member are the sole elements in contact with the worm screw when the contact member reaches the end of its travel relative to the resistive and conductive tracks. This method lacks the ability to reset the position of the ratchet member to the body portion.
U.S. Pat. No. 4,114,133 to Stephens discloses a worm gear drive and ratchet system in which a flexible, two-piece, toothed drive wheel is driven by a threaded drive shaft. The two-piece, toothed drive wheel comprises identical halves that are keyed and bonded into a fixed relationship. Upon reaching a stop, gear teeth embossed on the flexible drive wheel ratchet over the threads on the threaded drive shaft. This method does not provide adequate engagement to maintain positive location of the drive wheel and will not reset the position of the drive wheel.
U.S. Pat. No. 4,114,132 to DeRouen et al. teaches a lead screw type control for a miniature worm gear actuated potentiometer having a rotor with a main drive spur gear and a ratcheting drive rack gear. The invention contemplated rack gear teeth formed with a gentle ramp opposed to a more acute ramp such as to resemble a saw-tooth configuration in profile. The main drive spur gear is formed on less than the total of the rotor circumference. The ratcheting rack gear is mounted on a portion of the rotor circumference that does not include the spur gear. The main drive spur gear teeth are functional in a plane perpendicular to the plane in which the ratcheting rack gears are effective. Ratcheting is accomplished when the driving lead screw has driven the main-drive spur gear to one of its limits in either a clockwise or counterclockwise direction. At this point, if the same direction of rotation is continued, the ratcheting rack gear is urged into engagement with the driving lead screw by the force of a contact spring. As long as the same direction rotation of the driving lead screw is maintained, the engagement continues so as to permit continued ratcheting and thus rotation of the worm gear without causing damage to the movable parts of the worm gear actuated potentiometer. This device is complicated and has too many parts. Further, it relies on a spring to urge a ratcheting gear into engagement with the driving lead screw to prevent damage to the moveable parts.
U.S. Pat. No. 4,771,262 to Reuss teaches a drive means of a rotary potentiometer that is arranged to work trouble-free in the case of misalignment between the rotary axis of the rotating body and the rotary axis of the driving member. Teeth are arranged in the rotating, or rotary body, the crests of the teeth are directed to the rotary axis. The driving member is adjacent to the one flank of the one tooth and the one flank of the other tooth. The flanks of the teeth are involutely rounded off, so that the applied torque remains substantially constant in the event of any misalignment of the driving member axis with respect to the body axis. This method provides a means of driving a rotary potentiometer in a misaligned condition and does not provide a method of resetting the potentiometer in the event it over drives the limits.
U.S. Pat. No. 4,357,591 to Gray discloses a trimmer potentiometer or a like variable resistor, which has a worm gear driving an elastomeric worm wheel carrying a wiper contact to slide on an arcuate resistance element. The worm wheel has gear teeth formed on circumferentially-spaced rim sections projecting cantileverwise from an axially thick unbroken rim of a radial flange on the main body of the wheel, some sections having only a few gear teeth and being separated by other sections having larger numbers of gear teeth. The extent of rotation of the wheel is limited to less than 360 degrees, and the sections with few teeth are arranged to be in mesh with the worm at the limits of wheel rotation so as to give a de-clutching effect by inward deflection of these sections on over rotation of the worm. This method relies on yieldable teeth of an elastomeric worm wheel to give a de-clutching effect at the limits of the wheel rotation and does not provide a method of resetting the potentiometer.
U.S. Pat. No. 3,982,220 to Rozema et al. provides a variable resistance control containing a one-piece stamped mounting bracket having an integral collector ring for positioning a rotatable gear and having a pair of integral yokes for aligning a lead screw in driving relationship with the rotatable gear. Contactor constrained to rotate with the gear wipingly engages a resistance element. Each of the yokes comprises a pair of arms arcuately clinched around the lead screw to secure the lead screw to the mounting bracket. Integral with the gear are two outwardly extending resilient fingers for engagement with a respective one of the yokes to arrest rotation of the gear. Continued rotation of the lead screw after rotation of the gear has been arrested flexes the resilient finger toward the lead screw for driving the gear out of engagement with the lead screw. An aperture is provided in the lead screw for insertion of a shaft or tool for remote actuation of the control or for actuation of another control. This method appears to apply to both linear and rotary potentiometers and will protect over travel by disengaging the gear from the lead screw by driving the gear out of engagement without having the ability to reset the position of the gear to the travel of the potentiometer.
U.S. Pat. No. 4,672,858 to Langowski teaches a nut/clutch assembly for a rotary power screw, such as a linear actuator power screw, is disclosed in which the power screw is selectively rotated in one direction or the other, and in which the power screw has a pair of stops spaced apart from one another and which are rotatable with the power screw. A nut is provided having a central opening threadably engageable with the power screw, and the nut is threadably movable in axial direction along the power screw upon rotation of the latter. The nut has a circumferential groove thereon, and this groove has a base and a pair of spaced side walls. A collar is slidably received within the circumferential groove, and the collar is resiliently clamped onto the nut such that the collar frictionally engages the grooved base with a desired frictional force so as to permit the nut to rotate relative to the collar upon the nut being rotatably driven by the screw, and upon the collar being held in fixed rotary position. The collar is secured to a portion of its application in such a manner as to prevent rotation of the collar and so as to transmit linear force between the application and the power screw. The collar is somewhat narrower than the circumferential groove in the nut so that upon the nut moving axially along the power screw and upon the collar being coupled to the application, the collar resists axial movement of the nut and frictionally engages one of the side walls of the groove, thereby to hold the nut against turning with the power screw, and further to ensure axial movement of the nut together with the collar along the power screw. When the nut engages one of the stops, the nut is positively rotated with the power screw and the collar held by the application turns relative to the nut so as to serve as a clutch. This method does not reset the nut to its proper location. This device has many parts and requires close tolerances to function properly. Further, the nut rotates with the power screw, requiring a groove to transfer the motion to a stationary potentiometer. The interface between the nut and the potentiometer must have a bearing surface.
There are electromechanical devices such as limit switches to shut off the drive prior to the potentiometer reaching its limit, however, the nut would then need to be reset or positioned manually by a service repair person.